WO 03/71561 describes the preparation of magnetic nanoparticles by decomposition of a cobalt precursor at an elevated temperature and pressure in a H2 atmosphere. Iron nanoparticles have been synthesised using the same method by decomposing Fe[N(SiMe3)2]2 (see Margeat, O. et al. Prog. Solid State Chem. 2005, 33, 71). The iron nanoparticles are highly air-sensitive, resulting in an iron/iron oxide core/shell structure.
Shao et al. also report the air-sensitivity of iron nanoparticles, and oxidised amorphous iron nanoparticles at the surface to give a crystalline Fe3O4 shell (IEEE Trans. Magn. 2005, 41, 3388). Peng et al. (J. Am. Chem. Soc. 2006, 128, 10676) modified this method by introducing a controlled oxidation of iron nanoparticles, which were prepared from Fe(CO)5 using a similar method to that described by Shao et al.
WO 03/86660 describes the preparation of metal/metal oxide core/shell structures in which the shell is deliberately formed in a sequential reverse micelle synthesis. This method involves multiple steps and the use of a reducing agent. Lai et al. report a one-step synthesis to produce Cr/Fe2O3 core/shell nanoparticles by thermal decomposition of the respective metal carbonyl precursors (J. Am. Chem. Soc. 2005, 127, 5730).
WO 2004/060580 describes another form of core/shell nanoparticle comprising a (transition) metal core with a noble metal shell, which is prepared with methods involving multiple and sequential steps. Such nanoparticles are also described by Carpenter (J. Magn. Magn. Mater. 2001, 225, 17).
The mixing of iron and carbon to make steel has been studied for centuries. Iron carbides form in several different stoichiometries. Fe3C is the most stable and two others exist based around this structure, Fe7C3 and Fe5C2.
Bulk carbides are generally produced by heating the metal and carbon to very high temperatures, typically above 1000° C. Nanoparticles of iron carbide have mainly been formed in experiments in furnaces operated at 500-700° C. Other methods involve high energy techniques, including laser ablation.
Solution syntheses are typically cheaper, more energy effective and enable scale up. In addition, such syntheses may provide some control of the shape of the nanoparticles formed. Yu and Chow describe a solution phase synthesis of iron-iron carbide nanocomposite/iron oxide core/shell particles from iron pentacarbonyl in diphenyl ether at 257° C. under an argon, methane or acetylene atmosphere (Journal of Applied Physics 2005, 98, 114306). The results of this work show that, without a source of carbon in addition to the iron pentacarbonyl, the amount of carbide formed is less than about 5%.
Iron nitride exists in many different phases. Those most frequently reported in the literature are γ′-Fe4N, ε-Fe3N and α″-Fe16N2.
In steel making, nitriding leads to the formation of nitrides of iron and other elements. The process is similar to that of carburisation, but at a relatively lower temperature. The synthesis of iron nitride nanoparticles usually involves heating an iron precursor in flowing ammonia gas. These syntheses are generally physical methods operating at temperatures in the range of 300-1000° C. (see Li et al. J. Magn. Magn. Mater. 2004, 277, 641).
In recent reports, Huang and co-workers (J. Magn. Magn. Mater. 2006, 307, 198) describe the preparation of 12-18 nm ε-Fe3N nanoparticles by passing a mixture of Fe(CO)5 vapour and ammonia gas through a carrier liquid containing surfactant molecules at 180° C. This temperature is much lower than those employed in the conventional methods. The nanoparticles oxidise on the surface to give ε-Fe3N/Fe2O3 core/shell structures.
It is an object of the present invention to provide an improved method for preparing magnetic nanoparticles; and/or to go some way to avoiding the above disadvantages; and/or to at least provide the public with a useful choice.
Other objects of the invention may become apparent from the following description which is given by way of example only.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date.